The invention relates to inkjet inks relatively free of photoinitiator and to a method and apparatus for curing one or more such inkjet inks.
Inkjet printing is known wherein one or more inkjet inks are laid on to a substrate. It is also known to cure the inkjet inks using ultraviolet (UV), radiation, such radiation causing polymerization of the inks.
Inkjet inks typically require low viscosity. However, low viscosity typically leads to low reactivity. To increase the reactivity, one or more photoinitiators are often used in inkjet inks in concentrations typically between 5% and 10%. Photoinitiators are molecules that are excited by the UV radiation to form free radicals that start the polymerization process.
It is known that oxygen may interfere with any photoinitiated process. This is one reason UV curable inkjet inks typically contain a large amount of photoinitiator to enable them to be UV-cured in air. It is also known that the UV curing process can be improved by inertization of the curing environment, for example by spreading a blanket of relatively inert gas such as nitrogen. By relatively inert is meant inert compared to oxygen with respect to the inks, substrate and UV radiation
The use of common photoinitiators has the disadvantage that residues of the photoinitiators included in the ink may migrate out of the coating after curing. This is disadvantageous because these migrated residues cause contamination of the environment. Most photoinitiators, for example, are classified as toxic substances. Inkjet inks that include such photoinitiators typically are not suitable for products with direct food contact.
There is thus a need in the art for inkjet inks that are free or relatively free of photoinitiator, and a process for curing such photoinitiator-free inks.
Several forms of inkjet printing are known. One form is drop-on-demand inkjet printing using one or more piezoelectric inkjet head. One form of such printing is multi-color printing wherein several inks are laid in sequence, with one ink possibly on top of one or more other inks. One form is wet-on-wet printing wherein all inks are laid in one pass prior to curing. Furthermore, grey-scale inkjet printing also is known wherein each ink is laid in varying quantityxe2x80x94e.g., varying drop sizexe2x80x94to provide a grey-scale. Note that inkjet grey-scale printing is usually xe2x80x9cpseudo-grey-scalexe2x80x9d printing with halftone screens that use very small dots (xe2x80x9cmicro-screensxe2x80x9d). Furthermore, inkjet printing on various substrates other than paper is known. The term xe2x80x9cindustrialxe2x80x9d substrates herein means substrates other than regular printing paper and include plastics, foils packaging materials, and so forth, as well as paper. One application is decorative prints.
There is a need in the art for UV curable inks that are relatively free of photoinitiator and that are adapted for several inkjet printing processes, including drop-on-demand inkjet printing using one or more piezoelectric inkjet heads, in particular wet-on-wet inkjet printing suitable for printing on a wide variety of industrial substrates, in particular grey-scale inkjet printing on a wide variety of industrial substrates.
Radiation curable inks, e.g., UV-radiation curable inks are described in European patent publication EP 0882104 to inventors Caiger, et al. (published Oct. 27, 1999) entitled xe2x80x9cRADIATION CURABLE INK COMPOSITION,xe2x80x9d U.S. Pat. No. 5,104,448 to Kruse (issued Apr. 14, 1992) entitled xe2x80x9cJET PRINTING INK COMPOSITIONS AND METHODS,xe2x80x9d U.S. Pat. No. 5,275,646 to Marshall et al. (issued Jan. 4, 1994) entitled xe2x80x9cINK COMPOSITION,xe2x80x9d U.S. Pat. No. 5,623,001 to Figov (issued Apr. 22, 1997) entitled xe2x80x9cINK COMPOSITION AND A METHOD FOR USING SAME,xe2x80x9d International (PCT) patent publication WO 9624642 to inventors Schofield, et al. (published Aug. 15, 1996) entitled xe2x80x9cINK JET PRINTER INK COMPOSITION,xe2x80x9d and International (PCT) patent publication WO 9929788 to inventors Johnson, et al. (published Jun. 17, 1999) entitled xe2x80x9cINK JET PRINTER INK COMPOSITION.xe2x80x9d These do not address reducing or eliminating photoinitiator.
The need to have UV curable compounds that are relatively free of photoinitiator has been recognized before. However, the prior art on eliminating or substantially reducing the amount photoinitiator has not specifically addressed inkjet inks and curing such inks by UV.
U.S. Pat. No. 5,446,073 to Jonsson, et al. (issued Aug. 29, 1995) entitled xe2x80x9cPHOTOPOLYMERIZATION PROCESS EMPLOYING A CHARGE TRANSFER COMPLEX WITHOUT A PHOTOINITIATORxe2x80x9d describes a composition containing charge transfer complex from at least once unsaturated compound having an electron donor group and having an electron withdrawing group and being free of any photoinitiating compound that is polymerized by being subjected to ultraviolet light. Neither how to make or how to cure an inkjet ink relatively free of photoinitiator is described. Furthermore, the curing described is at UV wavelengths from an iron-doped lamp, i.e., using radiation rich in UVB. Aspects of the present invention include an ink for curing with a lamp rich in UVC and a curing method therefor.
U.S. Pat. No. 6,025,409 to Jansen (issued Feb. 15, 2000) entitled xe2x80x9cRADIATION CURABLE COATING COMPOSITIONxe2x80x9d relates to a coating composition comprising (a) a radiation-curable binder composition comprising an unsaturated compound having at least one maleate, fumerate, itaconate, citraconate or mesaconate group; (b) an unsaturated vinylether compound and a compound which forms a strong exciplex with (a) or (b); that is otherwise free of a photoinitiating compound. This describes a coating material, not an ink. A coatingxe2x80x94a varnish with no pigmentxe2x80x94is typically easier to cure than an ink that contains pigment. Pigment typically absorbs UV radiation. Furthermore. Jansen uses an excimer lamp at 172 nm and an iron-doped lamp. A 172 nm source would not work for a thick ink layer, i.e., a layer with pigment, only for a thick coating. Excimer lamps also are known to produce a dull matte effect. No mention is made in Jansen as to how to produce the low viscosity required for inkjet printing. Furthermore, excimer lamps are not practical for an inkjet printer because they require a very low level of residual oxygen to work, they are relatively expensive, they are not known to be reliable, and they have a short life. Thus, again, how to make and cure an inkjet ink relatively free of photoinitiator is not described.
U.S. Pat. No. 6,030,703 to Fan, et al. (issued Feb. 29, 2000) entitled xe2x80x9cRADIATION CURABLE COMPOSITIONS COMPRISING AN UNSATURATED POLYESTER AND A COMPOUND HAVING TWO TO SIX-PROPENYL ETHER GROUPSxe2x80x9d describes a composition suitable for free radical, radiation curable coating comprising at least one polyester having unsaturated backbone and at least one compound having two to six propenyl ether groups, and optionally, a free radical photoinitiator. While no photoinitiator is used, the material described is for a coatingxe2x80x94a varnish with no pigmentxe2x80x94that is easy to cure compared to inks that have pigment. The curing described in U.S. Pat. No. 6,030,703 is by EB rather than by UV.
U.S. Pat. No. 6,034,150 to Hoyle, et al. (issued Mar. 7, 2000) entitled xe2x80x9cPOLYMERIZATION PROCESSES USING ALIPHATIC MALEIMIDESxe2x80x9d describes aliphatic maleimides and methods using the same. Polymerization of compositions that include the compounds of the invention may be activated by irradiating the composition with radiation in the absence of a photoinitiator. However, there is no description of how to adapt this material to make inkjet inks. The ink invention described herein does not use such molecules.
U.S. Pat. No. 6,127,447 to Mitry, et al. (issued Oct. 3, 2000) entitled xe2x80x9cPHOTOPOLYMERIZATION PROCESS AND COMPOSITION EMPLOYING A CHARGE TRANSFER COMPLEX AND CATIONIC PHOTOINITIATORxe2x80x9d describes a radiation curable coating composition is provided and includes an effective amount of cationic photoinitiator, in combination with a charge transfer complex. One embodiment is a compound containing free radical photoinitiator of between 0.5% by weight to about 1% by weight of the total composition and cationic photoinitiator of between 1% by weight to about 3% by weight of the total composition, so that there is in one embodiment at least 1.5% photoinitiator. Furthermore, cationic photoinitiator has been found to be incompatible with the other ink materials used in drop-on demand inkjet printing. Thus, there still is a need for inkjet inks that are relatively free of photoinitiator and that do not use cationic photoinitiators.
Thus there still is a need for a UV curable inkjet that is relatively free of photoinitiator, that can be completely cured in an acceptable amount of time. There also is a need for a method and apparatus for curing such an ink. In particular, there is a need for a UV curable inkjet ink suitable for drop-on-demand (DOD) inkjet printing. In particular, there is a need for a UV curable inkjet ink suitable for drop-on-demand (DOD) wet-on-wet inkjet printing
One embodiment is a method of polymerizing one or more inkjet inks having a viscosity and superficial tension suitable for inkjet printing. The method includes depositing the one or more inks on the substrate using one or more inkjet heads in an inkjet printer, irradiating the deposited inks with UV radiation relatively high in UVC radiation (xe2x80x9cUVC-rich radiationxe2x80x9d), the UVC-rich radiation produced by a first UV source, the irradiation while the substrate is moved relative to the first UV source. The method further includes depleting the region close to the substrate surface of oxygen such that the between region between the first UV source and the laid out inks is relatively free of oxygen during the irradiating to increase the amount of UVC reaching the inks and to reduce the lowering caused by oxygen of the effectiveness of any photoinitiator in the ink. The inks are relatively free of photoinitiator.
Also described herein is an inventive UV curable inkjet ink comprising between 20% to 40% by weight of monofunctional acrylate between 35% to 55% by weight of difunctional acrylate, between 5% and 15% of trifunctional acrylate, between 5% and 15% highly reactive acrylate, up to 5% of one or more pigments, and no more than 0.5% of photoinitiator, which may be a single photoinitiator or a blend of two or more photoinitiators.
One embodiment further includes a relatively small amount, e.g., 0.5% to 1.5% by is weight, of one or more flow promoting agents for rheology control. The amount of flow promoting agent in one embodiment is approximately in the same ratio to that of the highly reactive acrylate.
Another embodiment includes substantially no photoinitiators.